1. Field of the Invention
The invention relates to the calibration and maintenance within calibration of the critical performance parameters of the individual transmit/receive operating circuits of each antenna element of a phased array radar and to a novel adjustable and interchangeable transmit/receive sub-assembly providing the operating circuits for specific antenna elements, by means of which a self monitoring/calibration process may be carried out.
2. Prior Art
In a conventional phased array radar system, power generation is accomplished through the use of a power tube or amplifier and then distributed to the individual radiating elements through a transmission line network. Care must be taken to insure that all individual transmission line paths are of the same or known line length to accomplish beamsteering and control over the desired frequency bandwidths. Line length calibration in service is usually not necessary.
In a solid state radar, a low power exciter usually generates the carrier of the transmitted signal. The exciter output is often modulated in amplitude or phase, including pulsing, to generate radar signals of low power. These low power signals are then distributed to an array of power amplifying modules each arranged to drive an antenna element of the phased array.
During transmission, it is essential that the power amplifiers in the modules retain phase coherence between themselves in order for the antenna pattern to be as specified. For some applications "weighting" of the power amplifier is used to reduce side lobes. The power level of the power amplifiers is also important for system performance for both weighted and unweighted transmission.
During reception, similar constraints are placed on the receiver function. In a phased array radar system, each antenna element is provided with a low noise amplifier. For the received "beam" to be properly formed, particularly for monopulse operation, where both sum and difference beams are formed, each low noise amplifier associated with each antenna element should process the signal with the same phase response and amplify it to the same degree.
In a customary phased array radar system, the need to meet a specific power gain or phase requirement applies to each of the several thousand operating circuits, each circuit associated with an element of the array. Accordingly, any correction must be efficient for large numbers of potential errors.
Radars operating at frequencies above 3 Gigahertz require a "high frequency" bulk material for their active devices, favoring use of a "MMIC" format. At these frequencies, active devices using silicon bulk materials become significantly less efficient than devices using higher frequency bulk materials such as Gallium Arsenide. At the same time, the actual sizes of the features of both active and passive components decreases, making it practical to integrate both the active and passive components on a single monolithic circuit. This circuit format is called the "Monolithic Microwave Integrated Circuit" (MMIC).
The MMIC format capitalizes on the semi-insulating quality of GaAs bulk material which permits efficient passive devices and circuit runs in layouts which are of controlled dimension using a photolithographic approach. The result is a very compact circuit construction.
Active devices in the MMIC format may be reproduced by a photolithographic process, and using certain new techniques they may be used to achieve adjustable gain in finely stepped increments. For instance, in the case of low noise amplifiers, stepped gain, and in the case of phase shifters, stepped phase.
An underlying fact in MMIC construction is that the effects of fabricational errors on circuit values are often greater than the circuit design can tolerate. For example, error in phase response may be substantially random in certain MMICs. Gain, however, is more predictable although still excessively variable for some applications. Thus, assuming an unacceptable error in a critical property due to manufacturing--or due to aging--the incremental property of MMICs suggests a way to achieve more exact circuit values in the operating circuits.